An MRI apparatus for obtaining a tomogram of a human body utilizing the nuclear magnetic resonance (NMR) phenomenon is used widely as a means for performing a diagnostic medical procedure. The MRI apparatus requires a static magnetic field generating device for generating a static magnetic field having a uniform magnetic field strength in the space where an object to be examined is inserted (hereinafter referred to as an examination space or imaging space) so as to accurately show an image of the inside structure of an examined portion of the object.
Since a uniform static magnetic field can be obtained within a solenoid coil having an infinite length, many of the static magnetic field generating devices used in an MRI apparatus employ a magnet structure, including a solenoid coil having a long, narrow cylindrical shape for generating a magnetic field with high strength and high uniformity in a space having a predetermined size, and a shim mechanism for further improving the magnetic field stability of the static magnetic field generated by the solenoid coil.
In an MRI apparatus that performs an examination on an object that is inserted in the long space within the cylindrical coil, the object is supported in the narrow examination space and is surrounded by the cylindrical coil around his/her body for a long time during examination. Therefore, the object has an oppressive feeling of being confined by the cylindrical coil, and, in some cases, the examination cannot be performed using an MRI apparatus having such a cylindrical coil on a person who has claustrophobia, on a baby, and the like, who cannot stand such an oppressive feeling.
Consequently, there has been developed in recent years an MRI apparatus including a static magnetic field generating device having a relatively low magnetic field strength, which employs a gantry structure in which the examination space is left open by providing an opening on the sides of the static magnetic field generating device, or a gantry structure in which the area in which the object is to be inserted, provided in front of the static magnetic field generating device, is widely opened.
The MRI apparatus having a open structure includes a permanent magnet or a resistive magnet as a magnet for generating the magnetic field. In the MRI apparatus using a permanent magnet or a resistive magnet, the S/N ratio of the image obtained in a high-speed mode is deteriorated in comparison with the MRI apparatus using a tunnel-shaped superconductive magnet. Also, it is not suitable for performing measurement of high-level functions, such as spectroscopic imaging and brain-function measurement.
Therefore, an MRI apparatus with an open magnet structure and including superconductive coils is being developed. The techniques mentioned in Japanese Patent Laid-open Publication JP 10-179546, JP 11-156831, and JP 11-197132 are examples thereof.
The above-mentioned open magnet including a superconductive coil can generate a magnetic field of 1.0-tesla magnetic field strength, which is five times the strength of the magnetic field generated by a conventional magnet using a permanent magnet or a resistive coil. If a fivefold magnetic field strength can be obtained, a fivefold NMR-signal strength can be obtained as well, whereby a S/N ratio that is sufficient even for high-speed imaging in real-time or the like can be achieved.
To open up the examination space of the magnetic field generating device containing the superconductive coils, as described above, a conventional superconductive coil with a tunnel shape is divided into two coils, the thus-divided coils are contained respectively in cryostats, and these cryostats are arranged opposite to each other with respect to the examination space.
However, it was revealed in the process of the inventor's developing a superconductive open-type MRI apparatus that, in a magnetic field generating device having the above-described structure, the superconductive coils contained in the cryostats are easily affected by vibration. That is, since the coil is contained in the cryostat as a unit in the magnetic field generating device including the conventional tunnel-shaped superconductive coil, the magnetic field generating device and the coil will vibrate as a unit when vibration is generated, whereby the position of the uniform magnetic field might shift slightly but the magnetic field strength is not affected critically. However, in the open-type magnet construction, the respective superconductive coils that are arranged opposite to each other for generating a static magnetic field vibrate differently, and it has been found that this difference in the respective vibrations causes the strength of the uniform magnetic field to fluctuate in some cases.
The reasons why the superconductive coils vibrate include induced vibrations from the surroundings of the magnet, vibration brought about by gradient magnetic field generating means driven in a pulsed manner, sound propagation due to vibration of the gradient magnetic field coils, and devices themselves that generate a vibration, such as the vibration of a helium refrigerator. Such vibrations cause the cryostats to vibrate, and, in turn, the vibrations propagate to the superconductive coils. Among those reasons for the vibration of the superconductive coils, the helium refrigerator and the gradient magnetic field generating means are a cause of transmission of vibrations to the cryostats in response to their operation. Another reason for vibration of the superconductive coils is that an electromagnetic force is exerted on the conductors of the gradient magnetic field coils when current is applied to the gradient magnetic field coils, and, in reaction, an electromagnetic force is exerted on the core of the superconductive coils.
Besides, the gradient magnetic field coils and the helium refrigerator operate periodically, causing fluctuation of the static magnetic field generated by the superconductive coils. If the width of such periodic vibration is small, its effect on the image can be disregarded. However, when the width increases to some extent, the fluctuation of the static magnetic field fluctuation cannot be disregarded, and it causes artifacts on the image and deteriorates the image quality.
That is, when the magnetic field fluctuates at regular time intervals (ω), measured NMR signals are modulated with a vibration frequency f (=1/ω). If the modulated signals are Fourier-transformed and converted into an image or spectrum, artifacts appear at positions, in real space where the vibration frequency f is reflected, that are spaced from the image. Even if the magnetic field fluctuation is slight, the fluctuation causes clear artifacts on the image and a pseudo-peak on the spectrum because of the regularity of the fluctuation.
Techniques for reducing vibration due to the helium refrigerator and the gradient magnetic field coils are described in U.S. Pat. No. 5,363,077 and in Japanese Patent Application No. 2000-203695. In those techniques, a mechanical deflection structure (spring) is inserted between a fixed body and a vibrated body, and the vibration energy of the vibrated body is absorbed by fixing the vibrated body to an object having a large mass (fixed body). On the other hand, there is a method of actively correcting magnetic field fluctuation components. For example, in the method described in U.S. Pat. No. 5,952,734 the components of the magnetic field error due to vibration are detected and the magnetic field correcting means is drive-controlled by a feedback loop. Further, the present inventors have proposed a technique of controlling magnetic field correcting means so as to accurately generate a correcting magnetic field which eliminates the detection error components generated by magnetic field fluctuation due to specific periodic mechanical vibration, such as vibration of the helium refrigerator (Japanese Patent Application 2000-34027.) This technique is very effective in reducing magnetic field fluctuation due to periodic vibration.
However, the above-described technique cannot compensate for the fluctuation of the static magnetic field due to resonance of the superconductive coils and/or its supporting mechanism. According to the inventors' research, it was found that the superconductive coils within an open-type static magnetic field generating device are periodically vibrated not only by mechanical vibration of the gradient magnetic field coils, but also the whole or a part of the divided structure of the superconductive coils is vibrated due to an electromagnetic force exerted on the superconductive coils by application of current to the gradient magnetic field coil in cycles of a specific value. Particularly, it has been found that the vibration width greatly changes when the superconductive coils and/or its supporting mechanism resonate with this vibration. Further, it was also found that the static magnetic field strength is fluctuated as this width increases.
The magnetic field fluctuation due to resonance of the superconductive coils cannot be disregarded since, in recent years, large energy is applied to the gradient magnetic field generating means in a short time period in new fast imaging methods, such as the fast spin echo (FSE) method and the echo planar method that have come to be frequently used. In these methods, the mechanical vibration period greatly changes because the vibration energy of the gradient magnetic field generating means is dramatically increased and the parameter setting range of the imaging conditions is expanded.
The present invention has been made in consideration of the above-described viewpoints, and a first object of the present invention is to provide an MRI apparatus that can achieve an improved reliability in obtaining an examination result by substantially canceling the static magnetic field fluctuation due to vibration of the open-type MRI apparatus without supplying energy from the outside.
A second object of the present invention is to provide a technique that is effective in substantially reducing or canceling fluctuation of the static magnetic field strength that is generated due to vibration of the open-type superconductive coil.
A third object of the present invention is to provide an MRI apparatus that can produce an image having a good image quality that is suitable for a doctor's diagnosis, on which artifacts are not generated, even if the open-type static magnetic field generating device is vibrated.
A fourth object of the present invention is to provide an open-type MRI apparatus that is able to maintain stability of the static magnetic field strength with a simple static magnetic field fluctuation reduction mechanism.
A fifth object of the present invention is to provide an open-type MRI apparatus in which a wide imaging space is established even when the static magnetic field fluctuation reduction mechanism is provided.
A sixth object of the present invention is to provide an open-type MRI apparatus in which a wide imaging space is established even when both a passive-shimming mechanism for improving the uniformity of the static magnetic field and a static magnetic field fluctuation reduction mechanism are provided.
A seventh object of the present invention is to provide a static magnetic field generating device by which the above-described first to sixth objects are achieved when the device applied to an MRI apparatus.